How to stretch 120 seconds of animation over 270 seconds?
Select the object and open the Dope Sheet. Set the frame scrubber (the green line) to the starting frame, press A to select all and then press S to scale the keyframes apart up to the required frame. This should stretch the animation to whatever length you want. If you have multiple keyframes, Blender will even these out for you. You can also hold Shift when scaling to snap in small increments.



Note of course that since you are making a short animation longer, it will most likely playback slower.